1. Field of the Invention
The present invention relates to a band switchable type tuning circuit which is adapted to tune to television signals of a high band or a low band.
2. Description of the Related Art
FIG. 5 shows a conventional band switchable type input tuning circuit. Television signals of a VHF band and a UHF band are input to a VHF input tuning circuit 22 and a UHF input tuning circuit 23 via a trap circuit 21 which eliminates unnecessary signals such as FM broadcast signals.
The VHF input tuning circuit 22 has a tuning varactor diode 22a whose anode is grounded, four coils, that is, a high-band coil 22b, a low-band coil 22c, a low-band coil 22d and a high-band coil 22e which are sequentially connected in series to a cathode of the tuning varactor diode 22a and which are connected in parallel to the tuning varactor diode 22a, and a switch diode 22f is connected in parallel with the two low-band coils 22c and 22d. The high-band coil 22e of a low potential side is grounded by a direct current (DC)-cutting capacitor 22g. And then, the television signals are input to a connection point of the two low-band coils 22c and 22d. Further, a connection point of the cathode of the tuning varactor diode 22a and the high-band coil 22b serves as an output terminal of the VHF input tuning circuit 22.
Also, a connection point of the high-band coil 22e and the DC-cutting capacitor 22g is grounded via a resistor 22h and is connected to a power supply terminal B via a resistor 22i. A power supply voltage of 5V is applied to the power supply terminal B. And then, the power supply voltage is divided by the resistors 22h and 22i, and the divided bias voltage is applied to an anode of the switch diode 22f via the high-band coil 22e and the low-band coils 22d and 22c. In addition, a cathode of the switch diode 22f is connected to a band switching terminal Sw via a resistor 22j. To the band switching terminal Sw, a switching voltage of a high level (5 V) or a low level (0 V) is applied. Further, the cathode of the tuning varactor diode 22a is connected to a tuning voltage terminal Tu via the high-band coil 22b. To the tuning voltage terminal Tu, a tuning voltage is applied.
In a stage next to the VHF input tuning circuit 22, a VHF radio frequency amplifier 24 is provided. The VHF input tuning circuit 22 and the VHF radio frequency amplifier are coupled by a coupling varactor diode 25. The coupling varactor diode 25 has the same characteristic as that of the tuning varactor diode 22a. A cathode of the coupling varactor diode 25 thereof is connected to the cathode of the tuning varactor diode 22a. Further, an anode thereof is grounded via a resistor 26 and is connected to an input terminal of the VHF radio frequency amplifier 24 via a DC-cutting capacitor 27.
The UHF input tuning circuit 23 has a coil 23a whose one end is grounded, and two tuning varactor diodes 23b and 23c of which cathodes are connected to each other and which have the same characteristic. An anode of the tuning varactor diode 23b is connected to the other end of the coil 23a and an anode of the tuning varactor diode 23c is grounded via a DC-cutting capacitor 23d and a resistor 23e. And then, a connection point of the coil 23a and the anode of the tuning varactor diode 23b is connected to the trap circuit 21 via a DC-cutting capacitor 31. Further, the cathodes of the two tuning varactor diodes 23b and 23c are connected to the tuning voltage terminal Tu while being connected to a UHF radio frequency amplifier 33 via a DC-cutting capacitor 32. For the UHF radio frequency amplifier 33, an FET is also used.
In a stage next to the UHF radio frequency amplifier 33, a UHF inter-stage tuning circuit 34 is provided.
In such a configuration, when the television signal of the VHF band is received, the VHF radio frequency amplifier 24 enters an enable state and the UHF radio frequency amplifier 33 enters a disable state. Meanwhile, when the television signal of the UHF band is received, the UHF radio frequency amplifier 33 becomes the enable state and the VHF radio frequency amplifier 24 becomes the disable state.
First, when the television signal of the VHF band is received, a voltage of the band switching terminal Sw becomes low level. If so, the switch diode 22f is turned on, and then the VHF input tuning circuit 22 is constructed by the two high-band coils 22b and 22e and the tuning varactor diode 22a. And then, a tuning frequency changes within the high band according to the tuning voltage to be applied to the cathode of the tuning varactor diode 22a. 
Meanwhile, when the television signal of the VHF band is received, the voltage of the band switching terminal Sw becomes high level. If so, the switch diode 22f is turned off, and the VHF input tuning circuit 22 is constructed by the two high-band coils 22b and 22e, the two low-band coils 22c and 22d, and the tuning varactor diode 22a. In this case, the tuning frequency changes within the low band according to the tuning voltage to be applied to the cathode of the tuning varactor diode 22a (for example, see Japanese Unexamined Patent Application Publication No. 2003-124786 (FIG. 5)).
In the above-mentioned configuration, the power supply voltage is divided by the resistors 22h and 22i and the divided bias voltage is applied to the anode of the switch diode 22f. Further, the cathode of the switch diode 22f is connected to the low level voltage (0 V) via the resistor 22j. Thus, when the switch diode 22f is turned on, a large current does not flow. Therefore, there are problems in that an on resistance of the switch diode is high and a Q of the VHF input tuning circuit 22 is lowered. Further, when the switch diode 22f is turned off, a reverse bias voltage becomes small. Therefore, there are problems in that a capacitance between the terminals of the switch diode 22f is high and the tuning frequency range of the VHF input tuning circuit 22 is limited.